iio: adc: palmas: use iio_event_direction for threshold polarity

Instead of having high_threshold > 0 as an indicator for upper threshold
event and lower threshold event otherwise, use enum iio_event_direction
instead. This is hopefully less ambiguous.

Signed-off-by: Patrik Dahlström <risca@dalakolonin.se>
Link: https://lore.kernel.org/r/20230408114825.824505-6-risca@dalakolonin.se


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>